1. Field of the Invention
The present invention relates to display apparatuses, such as flat panel displays, and to organic electroluminescent devices used for the display apparatuses.
2. Description of the Related Art
Recently, emissive devices usable for flat panels have been receiving attention. Examples of emissive devices include plasma emission display devices, field emission devices, and electroluminescent devices (hereinafter referred to as xe2x80x9cEL devicesxe2x80x9d).
Among them, with respect to an organic EL device in particular, it has been demonstrated in 1987 by T. W. Tang et al. that light emission with high luminance can be achieved by low-voltage DC driving using a layered thin-film structure including a luminescent metal chelate complex and diamine molecules, and research and development thereof has been carried out extensively. In low molecular organic EL devices, displays which emit light of a single green (G) color or which emit blue (B) and red (R) light in addition to green (G) light as area colors have been commercially available, and full color displays have also been under development.
When the organic EL devices are used for display apparatuses, they must be driven at as low a current and voltage as possible in view of the lifetime restrictions on drivers, etc. In order to control the luminance, a driving method using switching elements, such as thin-film transistors (TFTs), is believed to be most suitable (refer to Japanese Patent Laid-Open No. 8-241057 and its corresponding U.S. Pat. No. 5,786,796).
The organic EL device is a carrier-injection type emissive device which uses light emission occurring when electrons and holes are recombined in a luminescent layer. FIG. 1 is a sectional view which schematically shows a conventional organic EL device. In FIG. 1, numeral 11 represents a metal electrode, numeral 12 represents a luminescent layer, numeral 13 represents a transparent electrode, and numeral 14 represents a transparent substrate. In general, preferably, a metal having a small work function, such as aluminum, is used as the cathode, and a transparent conductor having a large work function, such as indium tin oxide (ITO), is used as the anode. The reason for using such a material for the anode is that light from the luminescent layer is passed through the anode toward the outside. A luminescent layer containing an organic compound is sandwiched between the two electrodes. The thickness of the organic compound layer is usually approximately several tens of nanometers.
In the conventional organic EL device, the transparent electrode 13 composed of ITO or the like is formed on the transparent substrate 14 composed of glass or the like, and the luminescent layer 12 containing an organic compound and the metal electrode 11 serving as the cathode are deposited thereon in that order. Light from the luminescent layer 12 goes out from the transparent substrate 14 side.
When a display apparatus is fabricated using the organic EL device having such a structure and using switching elements, such as TFTs, for driving, the pixel aperture ratio is decreased because the switching elements are formed on the transparent substrate 14. Therefore, an organic EL device has been disclosed in which a substrate for forming switching elements is also placed on the side of the metal electrode 11. In such a structure, because the switching elements and circuits are provided on the side of the metal electrode 11, it is possible to design the device without aperture ratio restrictions, and thus, a high-efficiency, high-definition display can be fabricated.
However, in the organic EL device described above, ambient light entering from the transparent substrate side is totally reflected by the metal electrode, resulting in a decrease in contrast due to reflected glare, etc.
On the other hand, Japanese Patent Laid-Open No. 9-127885 discloses a display device in which, by providing a quarter-wave plate and a linearly polarizing plate (circularly polarizing filter) on the outer surface of the transparent substrate 14, reflection of ambient light can be reduced, thus preventing the reflected glare. However, when such a circularly polarizing filter is used, it is not possible for almost half of the light emitted from the luminescent layer to pass through the filter, resulting in a decrease in luminance.
Additionally, a method is also disclosed in which an antireflection layer is provided on the cathode (Japanese Patent Laid-Open No. 2000-315582). FIG. 2 is a sectional view which schematically shows a conventional organic EL device having such a structure. In FIG. 2, the same elements are shown by the same numerals as those of FIG. 1. Numeral 15 represents an antireflection layer.
Since a transparent conductor is preferably used for the anode and a metal is preferably used as the cathode in view of the work functions, as described above, the antireflection layer is inevitably provided on the luminescent layer side of the cathode. In such a structure, the luminance is not decreased by the polarizing filter as in the case described above. However, since the antireflection layer 15 is provided between the metal electrode 11 and the luminescent layer 12, the work function must be taken into consideration so as not to hinder the electron injection from the metal electrode 11 to the luminescent layer 12, and also the antireflection layer 15 must be composed of a material which does not inhibit conductivity of the device. Therefore, strict restrictions are imposed on the material.
A device is also disclosed in U.S. Pat. No. 5,986,401 in which an antireflection layer is provided below an organic EL layer. In this structure, a transparent organic EL layer is formed and then an antireflection layer is placed therebelow. In one embodiment, the organic EL layer is deposited on a substrate with a planarizing layer therebetween, and in the other embodiment the antireflection layer is formed under a transparent substrate provided with the organic EL device.
As a result of investigation by the present inventors, it is assumed that in the structure according to U.S. Pat. No. 5,986,401, since the planarizing layer or the glass substrate is placed between the organic EL layer and the antireflection layer, interference of light occurs, and thereby the antireflection effect is effectively decreased. As an electrode for the transparent organic EL device, a transparent electrode composed of ITO is usually used. When an ITO film is placed on the transparent substrate composed of glass or the like, or on the planarizing layer, which is generally colorless, it is known that a peculiar light reflection effect is produced therebetween and the presence of the ITO film becomes noticeable. Therefore, further enhancement of the antireflection effect has been demanded.
The present inventors have discovered a structure in which a superior antireflection effect is exhibited based on the structure described above. It is an object of the present invention to provide an electroluminescent device in which glare due to reflection of ambient light is suppressed without decreasing the luminance.
In one aspect of the present invention, an electroluminescent device includes: a plurality of first electrodes formed on a substrate by patterning; an organic compound layer containing a plurality of luminescent sections: a second electrode, the first electrodes and the second electrode being substantially light transmissive; and a light absorption layer composed of an electrical insulator, the light absorption layer being placed in contact with the lower surfaces of the first electrodes and extending over the spaces between the adjacent first electrodes.
Preferably, the electroluminescent device is of a passive matrix type in which the first electrodes are formed in a striped pattern, or of an active matrix type in which the first electrodes are pixel electrodes connected to drain electrodes of transistors placed on the substrate.
More preferably, the pixel electrodes are placed on the transistors with the light absorption layer therebetween.
Preferably, the light absorption layer is composed of a material in which a light absorbent is dispersed in a polymeric compound.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.